It's not a problem for us at all if you're after a custom size!
If it's a non-standard size (eg. 110 x 165mm), order the next largest size up (eg. 120 x 180mm) and put in the comments that you want it cut to the smaller size you need.
Your artwork must be supplied at the required finished size.
If you need help working out what size to order get in touch with firstname.lastname@example.org for guidance.